In the field of occupant protection for motor vehicles it has become more and more important in the last several years to adjust the triggering of occupant retention means, for example front airbags, side airbags, knee airbags, curtain airbags, etc., to the vehicle occupants in the deployment area of the said occupant retention means or even to suppress said triggering in order, on the one hand, to save on subsequent repair costs following an unnecessary deployment, for example in the case of an unoccupied seat, not to trigger an occupant retention means from the outset, and, on the other hand, in order not to put certain groups of persons, for example children or very small adults, at additional risk due to an unsuitable triggering behavior of the occupant retention means. It is therefore important not only to detect the presence of a person on the motor vehicle seat, but in addition even to determine classifying characteristics of said person, for example the body weight. Deserving of mention in this context is the crash standard FMVSS 208, compliance with which is increasingly required by vehicle manufacturers and which stipulates a classification of a person according to his or her weight in order, in the event of a collision, to adjust the activation of an occupant retention means if necessary in a suitable manner to the person detected.
Various devices are known for detecting the weight or the weight distribution of a person on a motor vehicle seat, for example pressure-sensitive sensor seat mats as described in the unexamined published German specification DE 101 60 121 A1, or force measuring devices which are mounted between the vehicle seat and the vehicle floor and in this way register the weight of a vehicle occupant. The sensors used in this case are, for example, capacitive sensors, illustrated for example in the unexamined published German specification DE 199 25 877 A1, column 7, line 30, and FIG. 1 in that document. However, use is also made of inductive sensors, such as described, for example, in the US patent U.S. Pat. No. 6,129,168 or the unpublished German patent application 10303706.3.
In the last-cited US patent specification the force measuring device comprises a housing (50) which is composed of a deflectable housing portion (56) and a rigid housing portion (52), as can be derived from the abstract pertaining thereto and also from FIG. 3. The displacement of the movable housing part (50) is registered by an inductive deflection sensor (52).
In particular in the preferred area of application of such a force measuring device, namely for occupant weight detection in vehicles, it has however been shown in companies' in-house development activities that when sufficiently and lastingly dimensionally stable housing materials are used, the spring constant of just one spring means in the last-cited form of just one housing portion is not sufficient to be able at the same time to apply measurement technology for registering the very large measurement range of between 0 and up to 1.2 t that is typically required by the vehicle manufacturers. For this reason, in the last-cited, not prior-published German patent application 10303706.3, a plurality of spring means (1; 1a; 1b; 31) are connected one behind the other inside the housing of the force measuring device within a particularly compact, stable and consequently particularly suitable rotationally symmetric force measuring device in order to lengthen the spring path and thereby reduce the spring constant, which is to say the spring hardness. This means, however, that a substantial amount of additional outlay is required during manufacture and for introducing the sequentially connected springs into the housing, as a result of which the manufactured product may become more expensive and therefore less attractive for a vehicle manufacturer.